bigquery: need additional documentation for tags

## Is your feature request related to a problem? Please describe.
The documentation on https://pkg.go.dev/cloud.google.com/go/bigquery seems doesn't tells about how to use tags on query result

## Describe the solution you'd like
I just want to add some line that we could use `bigquery` tags so the struct doesnt have to be exactly same with the query. E.g:
```
type Count struct {
YearCount int `bigquery:"year"`
NumCount int `bigquery:"num"`
}
```

## Describe alternatives you've considered
N/A

## Additional context
N/A